Method of and apparatus for slitting webs

ABSTRACT

Thin elongated webs are slit by circular slitter blades having razor sharp cutting edges into strips having straight line edges which are parallel to each other and which are held to a close tolerance width dimension. The web is carried across cutting edges of the slitter blades by a traveling support surface and is held thereon against longitudinal or transverse shifting movement relative to the traveling support surface. The slitter blades are rotated to distribute wear uniformly about the circumferences thereof.

Umted States Patent [151 3,695,131

Zimmermann [451 Oct. 3, 1972 METHOD OF APPARATUS FOR 2,786,673 T 3/1957 Bridenstine ..83/100 UX SLITTING WEBS 3,173,326 3/1965 Gulliksenu ..83/501 Inventor: Ernest F. Zimmerman, Buffalo .Perrault G lll.

Primary Examiner-Donald 0. Kelly Asslgneei AmPeX Corporation, Redwood city, Attorney-Fitch, Even, Tabin & Luedeka and Robert Calif. G [22] Filed: Nov. 9, 1970 [57] ABSTRACT [21] Appl. No.: 87,677

Thm elongated webs are slltby circular shtter blades having razor sharp cutting edges into strips having [52] US. Cl. ..83/56, 83/ 152, 83/422, Straight line edges which are parauel to each other and 83,43" 83/500 which are held to a close tolerance width dimension. Int. The web is carried across i g edges of the slitter [58] held M Search 35 5 336: blades by a traveling support surface and is held /4 thereon against longitudinal or transverse shifting movement relative to the traveling support surface. [56] References cued The slitter blades are rotated to distribute wear UNITED STATES PATENTS uniformly about the circumferences thereof.

1,939,925 12/ 1933 Schwartz ..83/ I00 10 Claims, 8 Drawing Figures PATENTEBuma 1912 IMVE'NTOR 77798) F Zmmem/dm Mil/4B4, lcdda, 4M 24. 4 TIM ATTYS.

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sum 3 or 3 4 EDGE CUT on ANGLE 200 205 5 oxmc DAMAGE INVENTOQ (Md M, 44a, Ta -d METHOD OF AND APPARATUS FOR SLITTING WEBS This invention relates to a method of and an apparatus for slitting webs or films into a plurality of strips or tapes having parallel longitudinally extending edges and more particularly to slitting very thin webs into tapes having close tolerance width dimensions.

As used herein, the term slitting refers to severing by drawing a web across a sharp blade edge in contrast to a shearing operation of the kind in which the severing is caused by and between two sharp edges positioned in closely adjacent planes and movable in these planes normal to the path of web travel.

The present invention is particularly useful for slitting very thin, flexible, lightweight webs, for example, synthetic resin films having a cross-sectional thickness of about 1 mil or a fraction of a mil although the apparatus may be used to slit webs having a greater thickness than 1 mil. Such thin webs are often produced at high speeds and at widths which are subsequently divided by severing into several strips each of which must meet rigorous tolerance dimensions. For example, the thin magnetic tape webs, e.g. a Mylar film substrate having a thickness of 0.00025 to 0.0005 inches with a thin coating of iron oxide thereon, which are used in tape cassettes or tape cartridges are desirably severed into individual tapes each having a predetermined width such as 150 mils. For accuracy or fidelity in recording and playback, the width of such magnetic tapes are desired to be held to close tolerances, e. g. plus or minus 0.002 inches and, in some instances, to a tolerance of plus zero and minus 0.002 inch tolerances. Such close tolerances are to be held for extremely long lengths of magnetic tape.

For economic production of inexpensive, thin magnetic tapes, the severing must take place at relative high speed and these dimensions should be maintained without necessity of replacing the slitter blades or servicing or maintaining the apparatus until after a large volume production of magnetic tape is achieved. Militating against long blade life is the fact that magnetic tapes have an iron oxide coating on the Mylar film substrate which coating is extremely hard, e.g. an MOI-IS of seven. Such hardness has heretofore been thought to wear sharp, razor blades and to prevent their use for severing magnetic tapes which are usually cut with a shearing apparatus. Even without the hard coating of iron oxide, presently known slitters appear to slit only limited lengths of thin, synthetic film before it is necessary to replace the slitter blades.

In addition to slitting webs to close dimensions and with parallel edges at high speeds and for long lengths of web, the process and apparatus should provide a quality edge which is relatively smooth and clean as contrasted with a rough and ragged edge. In many instances, magnetic tapes have been scrapped because of damaged material along the edges resulting from the severing operation. Such damage may be in the form of a rolled edge portion bent from the plane of the magnetic tape or an extruded marginal area of the substrate along the edges of the magnetic tape. Such rolled or extruded edges are formed when the very thin, flexible film bends or moves into the space between the shearing surfaces prior to the rupturing of the film. Also, the edge of tape may be uneven and ragged rather than having a smooth straight line edge because the cutting edges are nicked, dull or are not sufficiently preloaded. With these kinds of edge damage, it is often the case that the iron oxide coating has been peeled or scraped away from the substrate at the damaged edge.

When slitting webs into strips with prior art apparatus at high speeds, the straight line parallelism of the edges over long lengths is not held as closely as desired, and, in some instances, it appears that the edges are wavy, i.e., serpentine. It appears that, during the course of severing, either the slitter blades or the film or both wander, i.e., move transversely to the path of film travel and cause this wavy edge condition. Such curves are particularly undesirable in magnetic tapes wherethe use of straight line recording tracks and at fixed distances from the tape edges is desired.

Accordingly, an object of. the invention is to overcome the foregoing disadvantages of prior art methods of and apparatus for slitting and to provide a slitter apparatus and method for slitting thin flexible webs suchas magnetic tape films.

Other objects and advantages of the invention will become apparent from the detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a diagrammatic, perspective view of a slitting apparatus embodying the novel features of the invention;

FIG. 2 is a partially sectional view taken substantially along the line 2-2 of FIG. 1;

FIG. 3 is an enlarged, fragmentary view of a slitter blade slitting a web on a support;

FIG. 4 is a view of a good quality, straight line edge cut by the apparatus of FIG. 1; and

FIGS. 58 illustrate edge defects found with severing magnetic tape webs with prior art devices.

As shown in the drawings for purposes of illustration, the invention is embodied in a method of and apparatus. 11 for slitting a web or film 12 into a plurality of tapes or strips 14 having parallel, straight line edges 15 and a width dimension held to close tolerance dimensions, e.g., to several thousandths of an inch or less. The preferred method comprises the steps of continuously feeding the web 12 along a path of travel through a slitting station 17, holding the web against shifting transversely of the direction of web travel at said slitting station while transporting the same on a traveling support surface 19, penetrating said web with circular slitter blades 25 located at positions spaced transversely of the path of web travel and at predetermined distances apart, and rotating said slitter blades to bring successive cutting edges 27 on each slitter blade into slitting contact with said web as it travels through the slitting station thereby distributing wear along the circumferences of the slitter blades.

In the preferred embodiment of the invention, the slitter blades 25 are thin circular disks which may be characterized as a circular razors having a continuous circular edge 27 in contrast to a straight line edge. That is, where a typical razor slitter blade has only one straight slitting edge, the circular slitter blades 25 provide a continuous series of cutting edges 27 which are brought into slitting contact during the rotation thereof. By way of example only, each one thousandth of an inch about the circumference of the edge 27 may be designated as one cutting edge; and a 3 9/16 inch diameter slitter blade will have approximately 1 1,200 cutting edges. For magnetic tapes, it is possible by turning the slitter blades and distributing the wear to obtain about 2,000 feet of slitting for each cutting edge. Thus, for this example, a total of 22,400,000 feet of film may be slit by each slitter blade before being replaced.

Straight line and parallel edges are achieved by holding the circular razor blades and the web 12 against shifting transversely relative to one another, i.e., wandering, during the slitting operation. Preferably, the web 12 is clamped or locked to the support surface 19 by a vacuum means which produces a differential pressure with ambient atmospheric pressure on one side of the web 12 and less than atmospheric pressure on the other side of the web while the support surface 19 is traveling at the same speed as the web and also transporting the web. The illustrated support surface 19 is the outer cylindrical surface of a rotatable vacuum cylinder 33 the interior of which is connected to a vacuum pump (not shown). To hold the slitter blade edges 27 against deflecting or bending from planes normal to the path of film travel, the slitter blades are supported and backed by blocks 31 throughout most of the area and across most of the diameters thereof with only a small outer circumferential portion at the blades being unsupported. With both the web 12 and slitter blade edges 27 thus held in predetermined and fixed positions, the edges 15 of the tapes 14 are severed along straight and parallel line edges 15.

Referring now in greater detail to the illustrated apparatus, the elongated web 12 is fed about the underside of a guide rod 35 and upwardly to the outer, cylindrical support surface 19 of the vacuum cylinder 33 about which the web is partially wrapped. The illustrated web is severed into a plurality of tapes 14 of which the outer ones are waste strands 37 and 39 which are taken off about a guide means 41 in the form of a rod. The strands 37 and 39 are considered waste because they are not held to the width tolerance dimension and because their outer edges 43 and 45 will not have been slit to provide the clean, close tolerance nonwavy edge which is produced as a result of the slitting. Only three useable tapes 14 are illustrated herein. In commercial practice, a three inch wide web of magnetic tape is slit into 20 tapes of which 18 will be held to 150 mil width and the remaining two outer strands having the edges 43 and 45 being considered waste. The useable tapes l4 hence constitute a much greater percentage of the total width of the web 12 than is illustrated in FIG. 1. The useable tapes 14 are shown as traveling off about guide means in the form of rods 47 and 49 to suitable tape up reels (not shown).

The web 12 is preferably transported by and at the same speed as the support surface 19 travels with the web locked thereon against movement relative thereto either longitudinally in the direction of web travel or transversely thereto during slitting. Preferably, the support surface 19 is endless and in the form of a rotatable cylindrical surface on the vacuum cylinder 33, which, as best seen in FIG. 2, is mounted on a stationary frame 55 for rotation by drive means 53. More specifically, the cylinder 33 has integral end wall 57 which is fastened to a rotatable drive shaft 59 and is rotated thereby. The shaft 59 has an integral circular flange 61 with an end surface 63 abutted against an interior surface 65 of the cylinder end wall 57. A small diameter projection 67 on the end of the drive shaft projects into an aperture 69 at the center of the end wall. A screw 71 is threaded into the projection and forces a washer 73 against an outer surface 75 of the cylinder end wall and forces the end wall surface 65 tightly against the facing flange surface 63. Thus, an outer end of the cylinder 33 is secured to the drive shaft 59 and is held against moving relative thereto.

The vacuum cylinder 33 encircles and rotates about a hollow post 77 fastened to a stationary plate 79 of the main frame 55. The post 77 is provided with an axially extending bore 80 through which projects the drive shaft 59 into a hollow cavity serving as the vacuum chamber 61. At the outer free end of the post 77 is a bearing 81 which journals the outer end of the drive shaft 59 at a position which is closely adjacent the flange 61 at the outer end of the drive shaft to stabilize the drive shaft 59 and cylinder 33 against wobbling. The bearing 81 is seated in a seat 83 formed in an outer end wall 85 of the stationary post 77. The post is formed with an outer cylindrical wall 87 which is positioned closely adjacent to the revolving, interior surface 89 of the vacuum cylinder 33. The vacuum cylinder 33 is formed at its other end with an enlarged diameter, sealing flange which telescopes over a circular air seal 97 fastened by a ring 99 to the stationary post 77. The seal 97 may be a graphite filled teflon ring which provides a low friction contact surface for engagement with the revolving, interior flange wall 101 of the sealing flange 95. Another ring shaped, air seal for the drive shaft 59 is seated in a circular seat 102 in the post 77 at the cavity of the vacuum chamber 61 to seal against air leaking into the chamber 61 by flowing axially along the drive shaft 59. Thus, the vacuum chamber 61 is effectively sealed against inward air flow along the drive shaft 59 or the post 77 so that the low pressure provided in:the vacuum chamber 61 may be maintained to provide the pressure differential across the web 12 when the latter covers vacuum ports 103 which are in fluid communication with the vacuum chamber 61.

The illustrated form of vacuum ports 103 are small bores of circular cross section extending from the outer support surface 19 radially inwardly through the cylinder wall to the inner surface 89 of the cylinder 33. Preferably, a circumferentially extending series of ports are formed in spaced rings about the support surface 19, there being three such rings of ports 103 in this instance. As the cylinder 33 rotates, some of the ports 103 move from an ineffective position in which they are covered by an arcuate wall 105 of the post 77 to an effective position in which the ports are aligned with and in fluid communication with the. vacuum chamber 61. The extent of the arcuate wall 105 is preferably limited to less than l80 and on the lower side of the cylinder 33 so that a good wrap and grip of the web 12 about the top of the cylinder may be achieved prior and subsequent to engaging the slitter blades 25.

The vacuum chamber 61 is in fluid communication with a vacuum pump (not shown) by means of a passageway means including a passageway 106 in the post 77 leading from the vacuum chamber 61 to a fitting 107 having an internal bore in fluid communication with an attached vacuum line 108 which extends to the vacuum pump (not shown). Thus, the vacuum pump exhausts air from the vacuum chamber 61 and establishes the pressure differential across the web 12 which is covering those ports which are in operative positions in alignment with the vacuum chamber.

As stated above, the clamping of the web 12 to the support surface 19 causes the web to travel with the support surface at the same speed the support surface is traveling and prevents a slipping and sliding of the web relative to the support surface and thereby the possibility of a lateral shifting or displacement of the web in a direction transverse to the path of film travel. Such lateral shifting could result in the formation of wavy serpentine edges 14 with one of the waste strands 37 or 39 becoming thicker while the other becomes thinner. The web 12 is fed to the cylinder 33 by a web feed device (not shown) and the tapes 14 are taken from the cylinder 33 by take up reels which are adjusted to feed the tape at the nominal speed of web travel. These feed devices are adjusted so as to provide a counter balancing of forces on the web 12 and the tapes 14 so that the force for feeding the web 12 through the slitting station 17 is only from the cylinder 33. Once started to move in a path, the web 12 continues to travel, i.e., track, in a straight path without shifting laterally along the surface 19.

In this illustrated embodiment of the invention, the web 12 spans a series of annular grooves 109 formed in the support surface 19 and aligned to receive the blade edges 27 of the slitter blades 25. For thin webs of magnetic tape, each of the blade edges project (e.g., 0.010 of an inch) into a groove 109. The amount of penetration of the blade edges into the grooves 109 may be varied from this particular dimension, particularly when slitting different thickness or kinds of webs. As

best seen in FIG. 3, each blade edge 27 is spaced from side walls 111 of its groove 109 to prevent a shearing action from occurring between the blades 25 and a side wall 111 of the groove. That is, the web 12 is unsupported at the cutting edge and is held taut to span the grooves between groove side walls 111. The relative movement of a web across the sharp edges 27 of the slitter blades results in a cutting action similar to that achieved by causing a web to be pulled across an edge of a pointed razor blade. The edges of the tapes 14 are, as shown in FIG. 4, smooth and less jagged than the edges of a tape (FIGS. 5-8) cut with a prior art device.

The driving means 53 for turning the cylinder 33 and drive shaft 59 comprises a sheave 1 13 fixed to an outer end of the drive shaft 59. The sheave 113 is driven by a belt 115 extending to a sheave 117 fixed to a motor shaft 1 19 of an electric motor (not shown). To provide a relatively constant rotational speed and to damper fluctuations in speed, a flywheel 125 is keyed by key 127 to the drive shaft 59 adjacent the sheave 113. The flywheel is annular in shape and a nut 129 threaded on the threaded end 131 of the drive shaft forces the flywheel axially inwardly on the shaft to a position against the sheave 113. The latter has a sleeve 133 which projects axially inwardly along the drive shaft 59 toward a washer 135 seated in an opening 137 in the port 77. Thus, it will be seen that the belt 115 drives the sheave 113 to rotate the drive shaft 59 and cylinder 33 at a relative constant speed.

The distribution of wear is preferably distributed uniformly and continually by turning the slitter blades 25 about a axis 140 with the slitter blades disposed perpendicular to the film travel path at the slitting station 17. The axis 140 is also a longitudinal and turning axis of a support shaft 141 for a slitter head 143 mounted on the support shaft 141. In this instance, the support shaft 141 is joumalled at its opposite ends in bearings 145 and 147 which in turn are suitably mounted in spaced, parallel, stationary plates 149 and 151. The plate 149 is fixed to the stationary frame 55 while the plate 151 is supported by the plate 149. More particularly, studs 1S3 extend between the plates 149 and 151 with a threaded end on each stud threaded in the plate 149 and holding the plate 151 against standoff sleeves 155 which encircle the studs and are disposed intermediate the plates 149 and 151.

The slitter head 143 is driven by a motor drive means to turn the slitter blades 25 to distribute the wear about the circumferential surfaces thereof. The speed of rotation may be quite low, e.g., 5 rpm, or it may be increased considerably to approach the speed of film travel. The drive means 157 for turning the slitter head 143 comprises a sheave 159 fixed to an end of the support shaft 141 and a belt 161 extending to a sheave 163 driven by a motor means 165. The motor means is preferably a continually operated electric motor so that the wear is continually being distributed about the blades circumferences.

The spacing between slitting edges 27 and hence the control over the dimension of the width of the tapes 14 is controlled by a plurality of accurately dimensioned blocks 31 disposed between the blades 25 with flat, parallel faces 167 thereon abutted against the flat, parallel sides of the blades. The spacer blocks 31 are annular in shape with central openings 168 which allow telescoping of a series of spacer blocks on a central hub 169 of the rotatable support shaft 141. As previously explained, circumferential surfaces 171 of the spacer blocks 31 extend closely adjacent the edges 27 of the blades 25 but remain spaced from engagement with the web 12 when it is on the support surface 19.

When assembling the slitter head 143, a firstslitter blade 25 (the bottom slitter blade as viewed in FIG. 2) is telescoped on the hub 169 of the shaft 141 to abut an annular wall 175 of a flange 176 which is integral with the shaft. This first blade is then backed by a spacer block 31 telescoped on the shaft hub and this spacer block is followed by another blade. A stack of alternating blades and spacer blocks are thus made on the shaft hub. Each of the flat parallel faces of the spacer blocks and blades are wiped clean and positioned in flat, tight engagement with each other. Then, an outer retainer 179 is threaded on a threaded portion 181 of the support shaft to abut a wall thereof against an adjacent face of a spacer block 31; and the retainer is tightened against this adjacent spacer block to exert axially directed forces through the stack of spacer blocks and blades to lock them against the flange 175 of the support shaft and against turning relative to the shaft hub 169. A hexagon portion 185 may be integrally formed of the support shaft 141 to receive a wrench to hold the shaft 141 against turning when a torque is being applied to the retainer 179 to tighten the same. In this manner, the slitter blades 25 are spaced at predetermined positions from one another and are held against deflecting and with their slitting edges 27 spaced at the desired predetermined distances from each other.

By way of example only, the illustrated slitter blades 25 are formed of a non-chromium, high carbon alloy steel made with a fine grain. The preferred blades are a forged product with the blade being sharpened by a grinding operation to the approximate angle for the edge and then the edge being lapped to a fineness of 20 millionths of an inch. The preferred blades having circumferential edges which have no nicks or grooves therein larger than 0.0001 inch in size. The illustrated blades are about 3 9/16 inches in diameter and are about 0.030 inch in thickness between the illustrated spacer blocks 31 which are as thick as the tape being slit less blade thickness. Such blades will produce straight line, smooth edges 15 for magnetic tapes with no visible damage to the iron oxide coating when viewed at 350 power magnification, such as is diagrammatically illustrated in FIG. 4.

Some of the typical defects in the cut edges 200 of magnetic tapes 201 severed from webs with prior art devices will be described herein in connection with FIGS. 5 to 8. When the cutting edge is not perpendicular to the plane of the web, the cut edge is not normal to the flat sides of the web but is at an angle to' the normal. A line 203 in FIG. 5 indicates the top of the cut edge 200 while a line 205 indicates the bottom of the angled edge. This angle edge 200 also illustrates oxide damage at 207 wherein the iron oxide on the top of a Mylar substrate 209 is peeled away to reveal the substrate 209 along an irregular line 211 and small, discrete particles 213 of iron oxide remain visible on the substrate. The loss of iron oxide at the edge 200 interferes with the ultimate use of the magnetic tape and is to be avoided. Where the edge of the blade is dull or nicked, the edge 201 of the severed magnetic tape 200 may be ragged with pieces 215 of the film substrate projecting outwardly and forming an uneven edge appearance, such as illustrated in FIG. 6.

When the web 12 is severed between a pair of shearing surfaces which allow the web to bend or roll into a space between the shearing surfaces, the cut edge 201 may have an extruded appearance (FIG. 7) in which it appears as if the substrate material, Mylar, in this instance, is stretched and pulled until it ruptures along a ragged outer line 219. The extruded edge is devoid of iron oxide coating between the line 221 and inner line 223 in FIG. 7. In some instances, the substrate 209 at the edge appears to be rolled, i.e., curved downward along the edge with particles of oxide missing leaving a mottled appearance 225 along the rolled edge, as best seen in FIG. 8. The rolled edge is also usually caused by the material being pushed down into the space between a pair of shearing surfaces and bending from a plane generally normal to the shearing surfaces of a plane more parallel to the shearing surfaces. Often the rolled edge occurs because the shear surfaces are grossly misaligned or spaced apart for the thickness of the thin web being severed. Here again, the edge is damaged between an inner line 227 and outer slightly ragged line 229.

As an aid to understanding the invention, a brief projection of the illustrated apparatus will be given.

The illustrated web 12 is approximately 3 inches in width and is formed with a Mylar substrate with an iron oxide coating thereon and has a total cross-sectional thickness of less than 0.0015 mils. The film 12 is wrapped about the vacuum cylinder 33 and the vacuum cylinder 33 is preferably rotated at a relatively fast speed corresponding to a linear speed of, for example, 500 ft. per minute to provide a good production rate while the slitting head 143 is rotated in a counter rotation to distribute edge wear at a speed, for example, of 5 rpm. The web 12 is preferably fed to the slitting station 17 by a feeding device (not shown) and the tapes 14 are preferably wound or taken up by another feeding device (not shown). The forces exerted by these respective feeding devices are balanced to cancel one another out so that the transporting of the web 12 through the slitting station is only as a result of the web being carried by the support surface 19 of the vacuum cylinder 33.

As the vacuum cylinder 33 rotates, each of the series of vacuum ports 103 therein moves from a lower ineffective upwardly and in a clockwise direction, as seen in FIG. 1, into fluid communication with the vacuum chamber 61 which is connected through the vacuum line 108 to a vacuum pump to provide the lower pressure inside of the vacuum cylinder 33. Thus, the higher ambient pressure on the upper or outer side of the web 12 holds the web in tight engagement with the support surface 19 and prevents any relative slipping movement in either the longitudinal and transverse directions between the web and the support surface 19. This holding or locking of the web prevents the web from tearing rather than being slit.

With the web locked to the support surface 19, the web is carried by the support surface 19 into engagement with blade edges 27 which, as best seen in FIG. 3, project into grooves 109 in the support surface 19. In this illustrated example, the blades 25 project for a distance of about 0.010 inch into the grooves 109. The illustrated blades are made of steel with very fine razor cutting edges and each 0.001 inch on the circumference of the blade is considered an equivalent to one straight edge razor type of slitter so that for a l 1.2 inch circular edge there are approximately 11,200 cutting edges with each of the respective cutting edges having the ability to slit approximately 2,000 feet of film before becoming worn. Thus, each slitter blade 25 should be able to slit approximately 22,400,000 feet of web before being replaced or resharpened.

The web 12 tracks onto the support surface 19 and locks thereto by pressure differential forces in a uniform manner without the use of side edge guides or the like so that edges 14 of the cut tapes are straight lines rather than serpentine. Also, as the slitting blades 25 are rigidly supported against deforming or bending, the edges 14 remain straight without waves therein as a result of a blade bending. As the blades 25 are held in positions substantially normal to the path of film travel, the edges 15 of the tapes 14 are normal to the flat, broad surfaces of the tapes rather than cut on angle as illustrated in FIG. 5.

The edge quality of the tapes 14 is found to be closely approximating the illustrated straight line edge illustrated in FIG. 4 with the width of the strand being held to the close tolerance dimensions. The cut edges are also found to have relatively little iron oxide damage along the edges thereof which would interfere with playback or recording operation on the iron oxide coating of the strand.

Thus, it will be seen by the foregoing that the present invention provides a simple and effective slitter ap paratus which slits magnetic tapes or other thin films into strands having clean, quality edges with the width of the strands being held to close tolerances. The apparatus is capable of producing quality edges for long lengths and periods of time without having to replace the slitter blades prematurely.

I claim:

1. A method of slitting a web of film-like material into a plurality of strips having closely dimensioned widths and parallel straight line longitudinally extending edges, said method comprising the steps of: continuously pulling the web along a path through a slitting station at a predetermined speed and under tension in a predetermined direction of travel, holding the web against shifting transversely of the direction of web travel while transporting the web on a traveling support surface, penetrating said tensioned web with un-interrupted continuous razor cutting edges on slitter blades located at positions spaced transversely of the path of web travel spaced from any cooperating shearing surface and at predetermined distances apart from each other, and pulling said web across said razor cutting edges to cause the slitting of the web, and shifting portions of said razor cutting edges into and from engagement with said web to distribute wear across said un-interrupted, continuous edges as said web continues to travel.

2. A method in accordance with claim 1 in which said razor cutting edges are circular and have edges with grooves therein less than 0.0001 inch in depth and in which said web is comprised of a synthetic resin substrate having a coating of ferrous oxide thereon for magnetic recording purposes, and further comprising the steps of establishing a pressure differential across said web to hold the same against said traveling support surface while said slitter blades slit said web into a plurality of magnetic tape strips, and rotating said razor cutting edges to distribute wear.

3. A method in accordance with claim 2 including the step of moving said traveling supporting surface and said web at the same speed and rotating said razor edges at a speed substantially slower than said speed of web travel.

4. A method in accordance with claim 3 including the further step of holding web to span openings in said traveling support into which said slitter blades project so that the film is slit only by the drawing of the film across the cutting edges of the slitter blades and is free of any cooperating shearing surface.

5. An apparatus for slitting a web of film-like material into a plurality of strips having closely dimensioned widths and parallel straight line longitudinally extending edges, said apparatus comprising means having a traveling support surface for transporting the web along a predetermined path of travel through a slitting station, means for holding said web against said traveling support surface during the slitting operation, a slitting head having slitter blades uninterrupted, continuous razor edges free of any cooperating shearing surface and free for penetrating said web and mounted for rotation about an axis spaced from said web, said circular slitter blades having said edges rotatable in planes perpendicular to said web, means to pull said web across said razor edges to slit said web, and means to rotate said slitting blades to successively turn the circumferential edges of said slitting blades into slitting contact with said web to distribute wear across said razor edges when a web travels through said slitting station.

6. An apparatus in accordance with claim 5 in which said traveling support surface is an endless surface and in which means are provided for driving said endless surface at the same speed said web travels through said slitting station, and said means to rotate said slitting blades turns said edges at a speed substantially slower than said web speed.

7. An apparatus in accordance with claim 6 in which said means for holding the web against said endless support surface includes mean for establishing a pressure differential on opposite sides of the web and said endless support surface, said pressure differential acting to force the web tightly against said endless support surface to prevent relative motion between the portion of the web thereon and said endless support surface.

8. An apparatus in accordance with claim 7 in which said endless surface is a rotating cylindrical surface and in which the web is partially wrapped thereabout, a vacuum chamber is provided within said cylindrical surface with a pressure therein which is less than ambient pressure and in which openings are provided in said surface providing fluid communication between the vacuum chamber and the exterior cylindrical surface on which said web is transported, said web covering said openings and causing a pressure differential to be established between the ambient on one side of the web and said openings and the vacuum chamber.

9. An apparatus in accordance with claim 8 in which spaced walls in said cylindrical surface define annular grooves therein and in which said walls defining said grooves are spaced from said slitter blades, said slitting being accomplished with the movement of the web across the cutting edges of said slitter blades.

10. An apparatus in accordance with claim 9 in which supporting blocks are positioned between said slitter blades to space said slitter blades at predetermined distances apart in a direction transverse to the direction of web travel, said supporting blocks extending radially outwardly to adjacent the cutting edges for said slitter blades to hold the same against deflecting from said planes normal to said web. 

1. A method of slitting a web of film-like material into a plurality of strips having closely dimensioned widths and parallel straight line longitudinally extending edges, said method comprising the steps of: continuously pulling the web along a path through a slitting station at a predetermined speed and under tension in a predetermined direction of travel, holding the web against shifting transversely of the direction of web travel while transporting the web on a traveling support surface, penetrating said tensioned web with un-interrupted continuous razor cutting edges on slitter blades located at positions spaced transversely of the path of web travel spaced from any cooperating shearing surface and at predetermined distances apart from each other, and pulling said web across said razor cutting edges to cause the slitting of the web, and shifting portions of said razor cutting edges into and from engagement with said web to distribute wear across said un-interrupted, continuous edges as said web continues to travel.
 2. A method in accordance with claim 1 in which said razor cutting edges are circular and have edges with grooves therein less than 0.0001 inch in depth and in which said web is comprised of a synthetic resin substrate having a coating of ferrous oxide thereon for magnetic recording purposes, and further comprising the steps of establishing a pressure differential across said web to hold the same against said traveling support surface while said slitter blades slit said web into a plurality of magnetic tape strips, and rotating said razor cutting edges to distribute wear.
 3. A method in accordance with claim 2 including the step of moving said traveling supporting surface and said web at the same speed and rotating said razor edges at a speed substantially slower than said speed of web travel.
 4. A method in accordance with claim 3 including the further step of holding web to span openings in said traveling support into which said slitter blades project so that the film is slit only by the drawing of the film across the cutting edges of the slitter blades and is free of any cooperating shearing surface.
 5. An apparatus for slitting a web of film-like material into a plurality of strips having closely dimensioned widths and parallel straight line longitudinally extending edges, said apparatus comprising means having a traveling support surface for transporting the web along a predetermined path of travel through a slitting station, means for holding said web against said traveling support surface during the slitting operation, a slitting head having slitter blades uninterrupted, continUous razor edges free of any cooperating shearing surface and free for penetrating said web and mounted for rotation about an axis spaced from said web, said circular slitter blades having said edges rotatable in planes perpendicular to said web, means to pull said web across said razor edges to slit said web, and means to rotate said slitting blades to successively turn the circumferential edges of said slitting blades into slitting contact with said web to distribute wear across said razor edges when a web travels through said slitting station.
 6. An apparatus in accordance with claim 5 in which said traveling support surface is an endless surface and in which means are provided for driving said endless surface at the same speed said web travels through said slitting station, and said means to rotate said slitting blades turns said edges at a speed substantially slower than said web speed.
 7. An apparatus in accordance with claim 6 in which said means for holding the web against said endless support surface includes mean for establishing a pressure differential on opposite sides of the web and said endless support surface, said pressure differential acting to force the web tightly against said endless support surface to prevent relative motion between the portion of the web thereon and said endless support surface.
 8. An apparatus in accordance with claim 7 in which said endless surface is a rotating cylindrical surface and in which the web is partially wrapped thereabout, a vacuum chamber is provided within said cylindrical surface with a pressure therein which is less than ambient pressure and in which openings are provided in said surface providing fluid communication between the vacuum chamber and the exterior cylindrical surface on which said web is transported, said web covering said openings and causing a pressure differential to be established between the ambient on one side of the web and said openings and the vacuum chamber.
 9. An apparatus in accordance with claim 8 in which spaced walls in said cylindrical surface define annular grooves therein and in which said walls defining said grooves are spaced from said slitter blades, said slitting being accomplished with the movement of the web across the cutting edges of said slitter blades.
 10. An apparatus in accordance with claim 9 in which supporting blocks are positioned between said slitter blades to space said slitter blades at predetermined distances apart in a direction transverse to the direction of web travel, said supporting blocks extending radially outwardly to adjacent the cutting edges for said slitter blades to hold the same against deflecting from said planes normal to said web. 